Ligand binding and macromolecular associations are generally grouped under the rubric "Reversible Associations" to denote their dependence upon noncovalent interactions that are in dynamic equilibrium in solution. A surface plasmon resonance instrument (BIAcore) is requested to join our existing analytical ultracentrifuge in order to establish a shared facility for the quantitative analysis of reversible associations. The research interests of the application participants encompass a broad range of important biomedical problems including assembly of gene transcription initiation and regulatory complexes, anti-HIV drug design, ligand binding and allostery, immunology and neuroscience. These instruments will be housed in a location convenient to the users' laboratories and will be accessible to the entire Einstein research community through the Laboratory for Macromolecular Analysis & Proteomics. The PI and other participants in this proposal have a proven track record in the organization and administration of shared resources. The institution provides substantial support to the Laboratory for Macromolecular Analysis & Proteomics including support for the instrument specialist who is overseeing use of the existing analytical ultracentrifuge and who will take charge of the new instrument requested in this grant.